Heretofore, a lithographic printing plate precursor comprises a photosensitive resin layer provided on a support having a hydrophilic surface. As for the plate-making method thereof, the lithographic printing plate precursor is ordinarily subjected to open frame exposure (mask exposure) via a lith film and then removal of the non-image area with a developer to obtain a desired printing plate. However, with the recent progress of digitized techniques, a computer-to-plate (CTP) technique of directly conducting exposure process on the surface of the lithographic printing plate precursor by scanning according to digitized image data with highly convergent light, for example, a laser beam, without using a lith film has been developed. Photosensitive lithographic printing plates (lithographic printing plate precursors) adapted for the technique have also been developed.
As the lithographic printing plate precursors suitable for exposure with a laser beam, lithographic printing plate precursors having a polymerizable photosensitive layer are exemplified. In the polymerizable photosensitive layer, it is easily enable to increase sensitivity by appropriately selecting a polymerization initiator or a polymerization initiation system (hereinafter also simply referred to as an “initiator” and an “initiation system”, respectively) in comparison with other conventional photosensitive layers.
As the laser light source, for example, a semiconductor laser of 405 nm or 830 nm or an FD-YAG laser can be used. In recent years, a CTP system equipped with a semiconductor laser of 405 nm has become widespread in view of handling properties.
In the hitherto known plate-making process of lithographic printing plate precursor, after the exposure as described above, the step of removing the unnecessary photosensitive layer by dissolving, for example, with a developer is required. However, it is one of the subjects to save or simplify such an additional wet treatment described above. Particularly, since disposal of liquid wastes discharged accompanying the wet treatment has become a great concern throughout the field of industry in view of the consideration for global environment in recent years, the demand for the solution of the above-described subject has been increased more and more.
As one of simple plate-making methods in response to the above-described requirement, a simple development processing with an aqueous non-alkaline solution without using an aqueous alkaline solution has been proposed. In such a development processing, an aqueous solution having pH of 10 or less is ordinarily used. For instance, in U.S. Patent Publication No. 2004/0013968 is described a processing method of lithographic printing plate precursor which comprises preparing a lithographic printing plate precursor comprising (i) a hydrophilic support and (ii) an oleophilic heat-sensitive layer which contains a radical-polymerizable ethylenically unsaturated monomer, a radical polymerization initiator and an infrared absorbing dye, is hardened with infrared laser exposure and is developable with an aqueous developer containing 60% by weight or more of water and having pH of 2.0 to 10.0, exposing imagewise the lithographic printing plate precursor with an infrared laser, and removing the unhardened region of the heat-sensitive layer with the aqueous developer.
On the other hand, as for the lithographic printing plate precursor, it is ordinarily conducted to incorporate a plate-inspecting agent (a coloring agent, for example, a dye or pigment) into the photosensitive layer of lithographic printing plate precursor in order to easily check the image-forming property.
Among them, as the plate-inspecting agent incorporated into the photosensitive layer of lithographic printing plate precursor of photopolymerization type, a pigment is often used because the pigment has an advantage of not inhibiting the photopolymerization. In order to improve dispersibility of the pigment, it is conventional to coat the pigment with a polymer or to substitute the surface of pigment with an acid group, base group or the like. However, for the lithographic printing plate precursor of simple development processing type with an aqueous non-alkaline solution, the pigment in the non-image area can not be completely removed after the development and a problem arises in that the color remains in the non-image area and stain occurs during printing.